Inventories - Schedule of Effects of Change in Accounting Principle From LIFO to Average Cost (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2018
Statement of Consolidated Operations
Cost of goods sold	$ 2,120	$ 2,485	$ 6,489	$ 7,540
Provision for income taxes	95	260	361	569
Net (loss) income	(147)	195	(498)	666
Net income attributable to noncontrolling interest	74	201	324	467
Net (loss) income (H), Alcoa Corporation	$ (221)	$ (6)	$ (822)	$ 199
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ (1.19)	$ (0.03)	$ (4.43)	$ 1.07
Diluted	$ (1.19)	$ (0.03)	$ (4.43)	$ 1.06
Statement of Consolidated Comprehensive Income
Comprehensive income (loss)	$ (375)	$ 377	$ (856)	$ 871
Comprehensive income (loss) attributable to noncontrolling interest	(10)	156	250	230
Comprehensive income (loss) attributable to Alcoa Corporation	(365)	221	(1,106)	641
Consolidated Balance Sheet
Inventories	1,649		1,649		$ 1,819
Prepaid expenses and other current assets	245		245		320
Retained earnings	(252)		(252)		570
Noncontrolling interest	1,871		1,871		1,970
Statement of Consolidated Cash Flows
Net (loss) income	(147)	195	(498)	666
Deferred income taxes			59	(16)
Decrease (Increase) in inventories (H)			111	(286)
Change in Accounting Principle from LIFO to Average Cost [Member] | As Computed under LIFO [Member]
Statement of Consolidated Operations
Cost of goods sold	2,104		6,495
Provision for income taxes	92		348
Net (loss) income	(128)		(491)
Net income attributable to noncontrolling interest	70		305
Net (loss) income (H), Alcoa Corporation	$ (198)		$ (796)
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ (1.07)		$ (4.29)
Diluted	$ (1.07)		$ (4.29)
Statement of Consolidated Comprehensive Income
Comprehensive income (loss)	$ (356)		$ (849)
Comprehensive income (loss) attributable to noncontrolling interest	(14)		231
Comprehensive income (loss) attributable to Alcoa Corporation	(342)		(1,080)
Consolidated Balance Sheet
Inventories	1,471		1,471
Prepaid expenses and other current assets	236		236
Retained earnings	(455)		(455)
Noncontrolling interest	1,887		1,887
Statement of Consolidated Cash Flows
Net (loss) income	(128)		(491)
Deferred income taxes			46
Decrease (Increase) in inventories (H)			117
Change in Accounting Principle from LIFO to Average Cost [Member] | Effect of Change [Member]
Statement of Consolidated Operations
Cost of goods sold	16		(6)
Provision for income taxes	3		13
Net (loss) income	(19)		(7)
Net income attributable to noncontrolling interest	4		19
Net (loss) income (H), Alcoa Corporation	$ (23)		$ (26)
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic	$ (0.12)		$ (0.14)
Diluted	$ (0.12)		$ (0.14)
Statement of Consolidated Comprehensive Income
Comprehensive income (loss)	$ (19)		$ (7)
Comprehensive income (loss) attributable to noncontrolling interest	4		19
Comprehensive income (loss) attributable to Alcoa Corporation	(23)		(26)
Consolidated Balance Sheet
Inventories	178		178
Prepaid expenses and other current assets	9		9
Retained earnings	203		203
Noncontrolling interest	(16)		(16)
Statement of Consolidated Cash Flows
Net (loss) income	$ (19)		(7)
Deferred income taxes			13
Decrease (Increase) in inventories (H)			$ (6)
Change in Accounting Principle from LIFO to Average Cost [Member] | As Originally Reported [Member]
Statement of Consolidated Operations
Cost of goods sold		2,534		7,547
Provision for income taxes		251		569
Net (loss) income		155		659
Net income attributable to noncontrolling interest		196		475
Net (loss) income (H), Alcoa Corporation		$ (41)		$ 184
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic		$ (0.22)		$ 0.99
Diluted		$ (0.22)		$ 0.97
Statement of Consolidated Comprehensive Income
Comprehensive income (loss)		$ 337		$ 864
Comprehensive income (loss) attributable to noncontrolling interest		151		238
Comprehensive income (loss) attributable to Alcoa Corporation		186		626
Consolidated Balance Sheet
Inventories					1,644
Prepaid expenses and other current assets					301
Retained earnings					341
Noncontrolling interest					2,005
Statement of Consolidated Cash Flows
Net (loss) income		155		659
Deferred income taxes				(16)
Decrease (Increase) in inventories (H)				(279)
Change in Accounting Principle from LIFO to Average Cost [Member] | Effect of Change [Member]
Statement of Consolidated Operations
Cost of goods sold		(49)		(7)
Provision for income taxes		9
Net (loss) income		40		7
Net income attributable to noncontrolling interest		5		(8)
Net (loss) income (H), Alcoa Corporation		$ 35		$ 15
Earnings per share attributable to Alcoa Corporation common shareholders:
Basic		$ 0.19		$ 0.08
Diluted		$ 0.19		$ 0.09
Statement of Consolidated Comprehensive Income
Comprehensive income (loss)		$ 40		$ 7
Comprehensive income (loss) attributable to noncontrolling interest		5		(8)
Comprehensive income (loss) attributable to Alcoa Corporation		35		15
Consolidated Balance Sheet
Inventories					175
Prepaid expenses and other current assets					19
Retained earnings					229	$ 205
Noncontrolling interest					$ (35)	$ (35)
Statement of Consolidated Cash Flows
Net (loss) income		$ 40		7
Decrease (Increase) in inventories (H)				$ (7)